Automatic transmissions are known in which the operation of a planetary gear train is controlled to provide the desired output gear ratio. In these known arrangements, a number of friction devices, that is, clutches and brakes, are engaged and disengaged to control the relative rotation of the gears of the planetary gear train to provide the required gear ratio. A problem with known arrangements is the necessity to control the timing of the disengagement of a clutch or brake whilst substantially simultaneously engaging another clutch or brake to provide a change from one gear ratio to another gear ratio.